In a mixing bowl, combine ground beef, rice, egg, cilantro, cumin, garlic powder, salt, and pepper.
Form the mixture into small meatballs, about 1 inch in diameter.
In a large pot, heat a splash of oil over medium heat.
Add the diced onion, sautéing until translucent.
Add sliced carrots and celery, cooking for another 5 minutes.
Pour in the beef broth and diced tomatoes, bringing to a simmer.
Carefully add the meatballs to the pot.
Allow the soup to simmer for 25-30 minutes, ensuring meatballs are cooked through.
Adjust seasoning with salt and pepper as needed.
Serve hot, garnished with fresh cilantro.